Hello, I've searched the forum, but I couldn't find exactly what I was looking for. I'm the original owner of a 2002 Tacoma, 3.4L, 5 speed, w/ manual t-case for 4wd. I've got over 600K mi and have encountered an issue with the 4wd. I do all my own mechanical work and am adept at maintenance and repairs. I went to put the truck in 4wd the other day and the dash indicator did not come on. My truck has a manual t-case shifter. There are no mechanical issues with the t-case and I knew I was looking at a signal/actuator issue. Confirmed that the t-case 'switch' is functioning as it completes the circuit when the shifter is moved to 4H or 4L and is an open circuit when shift lever is placed in 2wd. My front diff add actuator is not engaging and not moving the shift fork to slide the collar over in the front diff. When everything is functioning you can hear it click over when you shift the transfer case lever into 4H/L. The front diff actuator is not vacuum operated--it is electronically controlled and has a 6 pin connector. I pulled the front diff actuator and bench tested the electric motor by applying 12v from an external source to pins 2 and 6. The motor spins beautifully. Reversed the polarity of the voltage source and the motor spins nicely in the opposite direction and the shift fork slides along just like you'd want. There are no mechanical issues with the front diff, either. However, when I checked the voltage on the harness side I don't get any signal across pins 2 and 6 (the 20amp fuse is ok). I have traced the harness as much as possible, but have not found anything suspect. Is there some 'controller' or relay I should be looking for?

Thanks for reading

 

Have you checked all the 4wd fuses? should be 19 on this fuse panel on the left side of your dash

 

Update: i've located a 'transmission control relay' and it has 12v coming in but no signal coming out. Looks like I'm gonna have to replace this relay for $120.

 

Almost every 4wd issue I’ve encountered has been a position switch not providing the right info to the 4wd module. Not the control module itself.

Before you pull the trigger on $120. Take each position sensor and test for continuity thru the whole range of the plunger. Not just 0% and 100%. If there is ANY dead spots it could effect the module’s reaction.

there should be one on the front diff and a up to 4 on the transfer case depending on which mode you have.

 

Thanks for the reply. The position switch on the t-case checked out okay as I mentioned in my original post. I am fortunate as I have a 5 speed manual with a manually shifted t-case so I do not have a lot of position switches (or 'Detection Switches' as they are referred to in the FSM diagrams). I have switch continuity with no degradation when the t-case shifter is placed in 4H or 4L and I have an open circuit when the shifter is in the 2H or Neutral position. I am unaware of a position switch on the front diff like you mentioned. I referenced a FSM wiring diagram after reading your reply and it seems to indicate an 'add detection switch' and an 'add limit switch', but these look like internal, integrated switches within the makeup of the front diff actuator itself. When I opened up the front diff actuator I could see the 'switch' mechanisms which are the wipers on integrated contact strips. I cannot visually locate and independent, external position switch on the front diff, but I will have another look. However, as I understand the wiring diagram from the FSM, the indicator switch at the front differential (whether it is integrated or independent) really only lights off the dash lndicator light and has no active control function. With that being said, I must confess that I don't know how to test my transmission control relay to see if it okay or malfunctioning.

 

Soooooo, another update for anybody who might be following my ramblings. It turns out the FSM does indeed have a test/diagnostic protocol for my situation. The transmission control relay ohms out per spec. However, the add actuator which I thought bench tested fine did not check out when I ohmed it out per the FSM. As best as I can tell, the internal limit switch function is faulty. By the way, many thanks to Yotatech for the sticky that has the FSM that I used. I will change out the add actuator and let you know how things go.